‘Geophagus’ brasiliensis and the other similar-looking species in their complex are not true Geophagus and will eventually be given their own genus. These fish are often large-growing, with true brasiliensis males reaching up to 10” and having the aggression of Central American cichlids. The exception there is probably ‘Geophagus’ iporangensis, which is essentially like a miniature brasiliensis that tops out at 4 - 5”. They are very difficult to tell apart, though, and they’re much less common than brasiliensis.
Geophagus sveni are part of the ‘true’ Geophagus of the surinamensis group. These fish are more peaceful (relatively) than the brasiliensis complex. They do best in loose shoals and are true to the eartheater name, sifting sand in search of food. These will still reach 6 - 8”.
The last fish you mentioned being available locally sounds like Rio Olimar, which is the collection location for Gymnogeophagus labiatus. These also are not geos, but gymnos. Most of these species are southern SA cichlids coming from temperature rather than climates — southern Brazil, Uruguay, etc.
Unfortunately as adults you probably won’t even have space for a single pair of brasiliensis in a 75 because of their size and aggression. There also isn’t much room for small groups of true geos unless you pick a small species (Geophagus sp. Red Head Tapajos or Geophagus parnaibae, which is rare). G. sveni and similar would probably appreciate a larger tank if you keep them in groups.